The present invention relates to a time synchronized measurement system, and more particularly to a system for time synchronizing a master device and a slave device. In the field of protocol measurement, protocol testers and monitoring apparatuses are used which send and receive protocol messages or only receive protocol messages. In order to be able to draw desired conclusions with regard to problems occurring during transmission, the protocol messages are given a time stamp when sent and received, and are stored. In this process different switching units may be responsible for different protocol messages so that with respect to the time stamp there are no problems when the different switching units are different measurement cards of a protocol tester or monitoring apparatus. A time basis, such as a test time or absolute time, may be defined within the protocol tester or monitoring apparatus. The test time forms a common time basis for all measurement cards of the device, i.e., the time stamps generated by the different measurement cards may be placed in relation to each other because they all refer to the same time basis. A problem arises when the corresponding measurement cards are implemented in different apparatuses, i.e., different protocol testers or monitoring apparatuses. Now the different protocol testers or monitoring apparatuses need to be synchronized with respect to a common time basis.
One protocol tester or monitoring apparatus acts as a master device and another protocol tester or monitoring apparatus acts as a slave device. The master device is equipped with a reference clock pulse-generating device and provides to the slave device a reference clock signal via a separate line. Both the master device and the slave device are equipped with a time measurement device, and the time measurement devices are synchronized with respect to the reference clock signal. While this makes the two time measurement devices run synchronously, the problem remains of setting the two time measurement devices to the common time basis, i.e., to a common test time or absolute time. Usually the master device and the slave device are connected via a communication network, such as an Ethernet network. A time server may be connected to the Ethernet network so that the master device and the slave device query information on the common time basis from the time server. What is problematic about this is that Information on the common time basis is related to a specific, previously agreed signal that emanates from the time server. When the agreed signal arrives at the master device or the slave device depends on the distance between the relevant devices and the time server and on the capacity, i.e., the traffic volume, on the network. For example, if a 10 MHz rectangular signal is used as the reference clock signal, then a resolution of 100 ns may be achieved. However because of the aforementioned effects, deviations in the ms range may already result between the time measurement devices of the master and slave devices relative to each other.
Another known procedure envisages equipping both the master device and the slave device with a GPS (Global Positioning System) receiver and synchronizing them for setting the common time basis with respect to the time information, i.e., UTC (Coordinated Universal Time), emitted with the GPS signal. However this procedure is expensive and does not take into consideration that in many environments there is no access to a GPS antenna.
What is desired is to develop a method for low-cost and reliable synchronization of a master device and a slave device with respect to a common time basis.